1. Field of the Invention
The present invention relates to the measurement of electrophysiological signals and in particular to the removal of power line interference in electrocardiograms (ECG).
2. Description of the Related Art
Power line interference (PLI) often damages ECG despite proper grounding, shielding and amplifier design, therefore various digital methods for removing 60 or 50 Hz PLI in ECG are known in the art. Some of these methods use external reference signal to facilitate the PLI removal, but require additional hardware and are impracticable in the battery supplied mobile ECG devices. Many of the methods that process solely the ECG use notch filters that involve noticeable distortions and ringing effect at the QRS-complexes in the ECG, that is why others methods of this group divide the treated ECG into “quiet” and “event” epochs and extrapolate the PLI estimates of a “quiet” to the following “event” epoch. The more accurate of these last methods are quite complex and suited only for offline usage, whereas the simpler ones can be realized in real-time, but their performance considerably worsens in the common occurrences of PLI with deviating from the nominal 60 or 50 Hz frequency and varying amplitude.
Another method was proposed by the author of the disclosed invention in a previously published paper (I. P. Mitov, “A method for reduction of power line interference in the ECG”, Medical Engineering & Physics, vol. 26, No. 10, December 2004, pp. 879-887). This method is for reduction of 50 Hz PLI in ECG sampled with 250 Hz that fit both the purpose and the ECG spectral range (with equal reason relevant for removing 60 Hz PLI are ECG sampling rates 240, 300 or 360 Hz).
The considered method uses adequate parabolic model of the pure ECG and estimates its inconstant part, that is then subtracted to obtain a residual signal containing PLI, by Least-Squares (LS) solving a system of properly derived equations. Moreover, the squared error of another such system is utilized to determine a weight (that ranges from one in the very “quiet” ECG segments to nearly zero at the large and sharp QRS-complexes) for the subsequent LS approximations.
The residual PLI signal is further subjected to discrete Fourier transform separating the PLI components related to the nominal power line frequency (NPLF) and its harmonics, and the initial estimates of all these PLI components are obtained by apposite averaging.
The separate series of initial PLI estimates are downsampled to the NPLF (thereby due to the “aliasing” phenomenon their frequency content appears near to the spectral origin), LS approximated using third order polynomials and the above-cited weights, and thus obtained final estimates of the PLI components related to the NPLF and its harmonics are subtracted from the corresponding ECG sample.
The considered method is very accurate, even if the PLI is with deviating from 50 Hz frequency and varying amplitude, but its procedures are to a certain extent unduly elaborated and relatively burdensome for real-time implementation in the electrocardiographic (ECG) devices.